For decades, capacitors have been an important and irreplaceable circuit element used often in electronic circuit designs. For example, capacitors are widely used in applications such as dynamic random access memory (DRAM), active and passive filters, analog-to-digital and digital-to-analog converters (A/D and D/A converters, respectively), operational amplifiers, radio and tuning circuits, oscillators and multivibrator circuits, time critical and time delay circuitry, noise reduction circuitry, charge pumps, power electronics, and many other diverse applications. A capacitor is defined in the simplest terms as a device having two conducting surfaces separated by an insulating material. A capacitor stores electrical charge, blocks the flow of direct current (DC), and permits the flow of alternating current (AC) depending essentially upon the capacitance of the device and the frequency of the incoming current or charge. Capacitance, measured in farads, is determined by three physical characteristics: (1) a thickness or average thickness of the insulating material separating the two conducting surfaces; (2) how much surface area is covered by the two conducting surfaces; and (3) various mechanical and electrical properties of the insulating material and the two conducting surfaces or electrodes.
Many forms of capacitors exist in the semiconductor industry. In the early development and marketing of the above mentioned technologies, parallel plate or parallel electrode capacitors were used as capacitance structures. The parallel electrode capacitor is a capacitor that has a planar top and a planar bottom conducting surface separated by a planar dielectric or insulator. The planar capacitor, although easy to manufacture, consumes a large amount of substrate surface area. Capacitors which have a large substrate surface area are not practical for use with current memory technology. Current memory technology requires capacitors which have small substrate surface areas in order to achieve competitive integrated circuit densities.
Another widely accepted capacitor structure is known as a trench capacitor. The trench capacitor is formed by first etching a deep well, trench, or hole in a substrate surface or a surface overlying the substrate surface. This trench or hole is used to form and contain two electrodes separated by an insulator, which is referred to as an inter-electrode dielectric. Other known structures such as double box capacitors, crown capacitors, fin capacitors, and other similar capacitive structures have been developed for memory cells.
In most cases, the capacitive structures discussed above are not easy to manufacture when the inter-electrode dielectric of the capacitive structure is made of an advanced material, such as a ferroelectric material or a high-permittivity dielectric. Capacitor electrodes that are made of advanced materials, such as conductive oxides and refractory metals, are also difficult to manufacture with the capacitive structures discussed above. In addition, all the capacitor structures mentioned above have a first electrode that is formed before the inter-electrode dielectric is formed. Therefore, the first electrode is exposed to heat cycles and various ambient conditions, such as an oxidizing ambient, which may damage or alter the performance of the overall capacitor structure.
As memory technology advanced, new materials, such as lead zirconium titanate and other ferroelectric materials were used to transform a dynamic random access memory (DRAM) cell into a non-volatile memory. In most cases where ferroelectric materials or high-permittivity dielectric materials are concerned, the capacitor structures described above are too complex or inadequate for use as a non-volatile memory. In many cases, non-volatile memories require inter-electrode dielectrics that are thicker than the inter-electrode dielectric thickness typically found in DRAM cells. Therefore, some capacitor structures, such as the trench capacitor and box capacitor, although ideal for DRAM applications, become more difficult to manufacture or increase in substrate surface area for non-volatile memories.